Add exception in check when high error code == 0

Although not commonly done, it should be possible to add error codes
together even if the high level error code is equal to zero.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 8819674..9c1198e 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -298,7 +298,7 @@
      * h = high level error code (includes high and module error codes).
      * l = low level error code.
      */
-    if ( high > -0x1000 )               // high < 0001000000000000
+    if ( high > -0x1000 && high != 0 )  // high < 0001000000000000
     {
         mbedtls_test_fail( "'high' is not a high-level error code",
                             line, file );